White supremacist hate groups have been losing ground in the United States for the past two to three years, according to officials of two organizations that attempt to monitor their activities. But a decline in membership and activities of various Ku Klux Klan and Nazi organizations appears to have pushed some of the most extreme members to a higher level of armed violence since 1983, according to these experts.

Now this higher level of violence may be waning.

Membership has been declining in these anti-black, anti-Jewish groups and effective law enforcement has kept some remaining members busy in court, say officials with the Southern Poverty Law Center and the Jewish Anti-Defamation League of B'Nai B'Rith.

Irwin Suall, fact-finding director of the league, estimates membership in Ku Klux Klan factions across the United States has been declining since 1981, when it reached about 10,000 to 11,500. His latest estimate of Klan membership: 6,000 to 6,500.

Those committing the violence recently ``were desperate,'' says Mr. Suall. Their organizations ``were getting nowhere.'' They intended, he says, to ``bomb and shoot their way into power. They were serious. It's crazy, but there it is,'' says Suall.

FBI officials believe they have caught most of the main suspects, including Bruce Carroll Pierce, Gary Yarbrough, and David Lane, who was denied bond April 5 on counterfeiting charges. Mr. Lane also is wanted for questioning in the January 1984 machine-gun slaying of Jewish radio talk show host Alan Berg in Denver.

Some FBI suspects are alleged to have committed violent acts that include bank and armored-car robberies, illegal possession of weapons, assaulting federal officers, and murder. The robberies apparently were committed to raise money to finance other extremist activities, according to an official of the Anti-Defamation League and a federal official who asked not to be named.

Experts familiar with the string of violence are concerned that other white supremacists sympathetic with aims similar to those apprehended, could turn also to violence.

Some ``potential right-wing terrorists are unbalanced enough'' to be drawn into violence by the publicity surrounding the recent arrests, says Bill Stanton of Klanwatch, a project of the Southern Poverty Law Center in Montgomery, Ala. The center has attempted to monitor Klan and Nazi activities for years and has helped bring some accused Klan members to court.

In testimony before a Senate subcommittee April 3, FBI Director William Webster said: ``The right-wing terrorist groups have demonstrated a substantial propensity for violence.'' He added: ``They are more dangerous and more significant than the old Klan groups from which they eminate.'' Mr. Webster went on to identify Nazi and other right-wing groups as well as the Klan.

These extremists, according to Webster, believe in racial and religious hatred, accumulate vast stores of weapons, and hide in sparsely populated areas.

FBI officials say they believe the nearly 30 extremists arrested in connection with the violence committed since 1983 are members of or connected in some way with a group called The Order. Both Klanwatch and the Anti-Defamation League believe The Order is connected with the Aryan Nations, a white supremacist group headed by Richard Girnt Butler, and based near Hayden Lake, Idaho.

On Nov. 25, 1984, 13 members of The Order signed a declaration of war against the United States, alleging that the US government is controlled by Israel. The declaration was distributed by the Aryan Nations, says Suall. He estimates The Order has no more than 100 members and the Arayan Nations no more than about 500. Klanwatch estimates are similar.

Nazi and neo-Nazi groups in the US have been ``withering away,'' according to Charles Wittenstein, of the Atlanta office of the Anti-Defamation League. But he is concerned about lingering sympathy for the aims of such groups, he says.

Suall attributes the decline of such groups to effective federal and local law enforcement and failure of the Klan leaders to deliver on promises to reduce job quotas, illegal immigration,and busing. In addition, the improving economy took away some of the frustrations of the members, he speculates.

Still, according to Klanwatch, white supremacists are utilizing two new tactics: cable television and computers. According to Klanwatch officials, longtime California Klan leader Tom Metzger produced a documentary titled ``Race'' that was aired on cable TV in San Diego and Dallas last year. Several hate groups have also established a national racist computer network to further communications between the groups, they say.